With increasing needs for reducing the weight of a car body and for assuring safety against collision, a die-quenching method is attracting notice as a manufacturing method for car components. The die-quenching method is a processing method wherein quench-hardening of a heated steel plate is performed by forming and simultaneously rapidly cooling it in press metal dies.
As a method for heating a steel plate for quench-hardening, an infrared heating method is attracting notice. The infrared heating method is such a method in which infrared rays are irradiated on and absorbed by a work thereby to heat the work.
In regard to a car component, such as a car body component, there is a demand to impart variations in strength within a single component to save the labor of welding a high strength component to a low strength component to manufacture a single target component. The so manufactured single component has an advantage that a high strength may be realized by the high strength part, while the low strength part is more susceptible to processing.
The following is a brief survey of certain Patent Literatures pertinent to the above mentioned background techniques.
In Patent Literature 1, there is proposed a technique of placing a plate member of a preset shape between a steel plate and an infrared lamp and in setting a distribution of heating intensity, of at least a part of a surface side of the steel plate not covered by the plate member so as to differ from that of its surface side covered by the plate member.
In Patent Literature 2, there is proposed an infrared heater in which infrared rays of high light intensity are irradiated on a certain region of the steel plate and in which infrared rays of lower light intensity are irradiated on its other region.
In Patent Literature 3, there is proposed an infrared heater in which the number of infrared lamps lighted is selected in response to a target heating temperature of the steel plate and in which the output intensity of each of the infrared lamps lighted is set at the same value.
In Patent Literature 4, there is proposed an infrared heater in which, to exercise region-wise control of heating states of a steel plate, the output of the lamps disposed in a preset row(s) of a matrix is reduced, while that of the lamps disposed in its other row(s) is increased.
In Patent Literature 5, there is proposed an infrared heater in which press-forming of a steel plate is commenced at a condition that a portion of a steel plate is heated by infrared rays to a temperature of an Ar1 transformation point as above whereas the temperature of the remaining portion thereof is in a range between room temperature and a temperature less than the Ar1 transformation point.